1. Field of Invention
The present invention relates to a photodiode with a type of optical waveguide.
2. Related Background Arts
A Japanese Patent Application laid open No. JP2013-110207A has disclosed a photodiode device that monolithically integrates a waveguide element and a photodiode element with a type of optical waveguide optically coupled with the optical element on a substrate common to the waveguide element and the photodiode element.
An optical communication system currently installed often configures, what is called, the wavelength division multiplexing (WDM) using wavelengths of 1528 to 1565 nm, which is often called as the C-band in the field. As the capacity of data to be transmitted explosively increases, an optical communication system using longer wavelengths, specifically, 1565 to 1612 nm, which is called as the L-band, has been investigate. However, a PD commercially available in the field often shows large temperature dependence, especially in optical sensitivity thereof, in the L-band. For instance, the optical sensitivity of a PD widely degrades at a temperature lower than 0° C. Accordingly, an advanced PD applicable to the L-band at a low temperature has been demanded.
Also, a future optical communication system inevitably requests a high speed operation exceeding 400 Gb/s and a complex modulation configuration such as multi-level amplitude modulation combined with a phase modulation. Accordingly, a photodiode applicable such high speed and high degree of optical modulation will be inevitably requested to be operable in higher frequencies. A waveguide PD is determined in frequency response thereof primarily by a time constant caused by parasitic and inherent capacitance and resistance attributed to structures around an absorption layer thereof and a carrier transport time within the absorption layer. In order to enhance the frequency response, both the carrier transport time and the time constant are preferably shorter. However, a thinned absorption layer to shorten the carrier transport time inevitably increases the capacitance thereof to increase the time constant. Accordingly, the field has also requested to make above two subjects consistent to each other.